the syntax for calling a method having no parameters on an object in Objective-C is as follows:
The syntax for calling a method with a number of parameters is:
In this example the name of the method is myMethodParam1:param2:param3: (each ':' acts as a placeholder for a parameter) and paramValue1, paramValue2, and paramValue3 are names of variables.
In Objective-C, each pair of square brackets delimit one method call. In your example there are two method calls: one to the objectAtIndex: method and one to the getValueInDollars method - thus you need two pairs of square brackets, like this:
I found a good basic tutorial about Objective-C here on Cocoa Dev Central. I recommend you work through it - it will make your life much easier.
subject: Accesing a method for a object in an array